1. Field of the Invention
The present invention relates to a magnetic core for a transformer used in a high speed data communication modem in a digital subscriber line (generally called xDSL), such as an asymmetric digital subscriber line (ADSL) and a very high-bit rate digital subscriber line (VDSL), and a composition of the same, particularly relates to a magnetic core for a transformer which exhibits excellent total harmonic distortion (THD) characteristics of a transmission waveform in a broad frequency band and a wide temperature range at the time of data communication and a composition of the same.
2. Description of the Related Art
In recent years, in the field of electronics, equipments have been required to be made smaller in size, smaller in thickness and improved in performance. Furthermore, in the field of communication equipment, the impedance of the primary coil in the pulse transformer for interfacing with the Integrated Services Digital Network (ISDN) etc. has to be increased in a broad frequency band to improve the transmission characteristics. Therefore, the primary coil is required to have a high inductance. The I.430 and other standards of the ITU (International Telecommunication Union)-T required that at least 20 mH be secured for the inductance of the primary coil of the pulse transformer.
Further, to reduce the size and thickness of electronic equipments in this way, the transformer has to be made smaller and thinner. Therefore, the necessary inductance is secured by increasing the magnetic permeability of the material of the magnetic core used for the transformer (Japanese Unexamined Patent Publication (Kokai) No. 6-263447, Japanese Unexamined Patent Publication (Kokai) No. 7-94314, Japanese Unexamined Patent Publication (Kokai) No.7-169612, Japanese Unexamined Patent Publication (Kokai) No. 7-211530, Japanese Unexamined Patent Publication (Kokai) No.7-278764, Japanese Unexamined Patent Publication (Kokai) No. 7-297034, Japanese Unexamined Patent Publication (Kokai) No. 8-85821, Japanese Unexamined Patent Publication (Kokai) No. 8-97045, Japanese Unexamined Patent Publication (Kokai) No.9-246034, Japanese Unexamined Patent Publication (Kokai) No. 10-12447, and Japanese Unexamined Patent Publication (Kokai) No. 10-335130), reducing the thickness of the transformer (Japanese Unexamined Patent Publication (Kokai) No. 7-201582, Japanese Unexamined Patent Publication (Kokai) No. 7-201585, Japanese Unexamined Patent Publication (Kokai) No. 7-201588, Japanese Unexamined Patent Publication (Kokai) No. 7-201589, and Japanese Unexamined Patent Publication (Kokai) No. 7-201590), mirror-polishing the bonding surface in the case of a split-type magnetic core such as an EE shape (Japanese Unexamined Patent Publication (Kokai) No. 9-246034), etc.
Even in a transformer of a modem used for connecting a telephone line transmitting an analog signal and a data terminal or computer handling a digital signal, a high inductance is similarly required (Japanese Unexamined Patent Publication (Kokai) No. 11-176643 and Japanese Unexamined Patent Publication (Kokai) No. 11-186044).
Furthermore, due to the rapid spread of the Internet, a demand has risen for communication systems enabling higher speed communication of large quantities of data compared with the ISDN, etc. New communication systems called the xDSL (x digital subscriber line) have been developed. The ADSL (asymmetric digital subscriber line) and the VDSL (very high-bit rate digital subscriber line) enabling a higher communication speed, etc. are included in the xDSL technique.
The xDSL requires a modem for converting a digital signal to and from an analog signal and the modem requires a transformer for insulating from a line. In the xDSL technology, transmission speed is much higher (16 kb/s to 9 Mb/s in ADSL and 1.5 Mb/s to 52 Mb/s in VDSL) and the operating frequency band is also high (30 kHz to 1.1 MHz in ADSL and 1 MHz to 10 MHz in VDSL). Therefore, the transformer used for a modem may be reduced in the inductance for raising the impedance compared with the conventional transformers for ISDN. Accordingly, the magnetic permeability of the material of the magnetic core of the transformer may be made smaller than that in the past and mirror-polishing of the bonding surfaces is not required even in the case of a split-type magnetic core such as an EE shape.
On the other hand, in high speed data communications using such xDSL, the transformer used in the modem for transmitting a data signal at a high speed has to have a small distortion of the transmission waveform and a small noise and has to be reduced in the rate of occurrence of transmission error, so that a material of the magnetic core having a small THD is sought. Therefore, the loss in the AC magnetic field (the eddy current loss, the hysteresis loss, and the residual loss) has to be reduced.
Note that the THD means the ratio of the total harmonics and the noise component with respect to the basic signal of the input data at the time of data communication as expressed by the following equation (1), so the THD becomes smaller the smaller the distortion of the transmission waveform or the smaller noise.
THD(dB)=20 log{(harmonics+noise)/(basic wave+harmonics+noise)}xe2x80x83xe2x80x83(1) 
As a material of a magnetic core for reducing the THD, the present inventors have developed an Mnxe2x80x94Zn based ferrite material described in the U.S. patent application Ser. No. 09/944,590 (corresponding to the Japanese Patent Application No. 2000-279101). The Mnxe2x80x94Zn based ferrite material filed as a prior application reduces the THD in a broad frequency band by specifying a range of its main composition, but there is a demand for one capable of reducing the THD in the broad frequency band from the other approach.
Furthermore, the Mnxe2x80x94Zn based ferrite material filed as a prior application reduces the THD in a wide temperature range (xe2x88x9240xc2x0 C. to +85xc2x0 C.) but becomes minimum at around xe2x88x9210xc2x0 C. and the THD worsens to about several dB on the higher temperature side and on the lower temperature side of the temperature showing the minimum value. Thus, it is desired to reduce the THD value in frequently used temperatures of around 25xc2x0 C.
Note that effects of the main composition and additives in Mnxe2x80x94Zn based ferrite materials are reported in various ways so far but there has been no report on a technique of reducing the THD in a broad frequency band and/or a wide temperature range.
For example, a technique described in the Japanese Unexamined Patent Application No. 4-336401 Publication is only for attaining high magnetic permeability in a high frequency range by limiting a main composition range and adding additives (SiO2, CaO, Nb2O5 and Bi2O3) by suitable amounts. Also, a technique described in the Japanese Unexamined Patent Application No. 10-50512 Publication is only for attaining high magnetic permeability in a high frequency range by limiting a main composition range, adding additives (SiO2, CaO, Nb2O5 and Bi2O3) by suitable amounts, limiting a range of an absolute value of a complex initial permeability in a broad frequency range and limiting a range of a value of DC resistivity. Furthermore, a technique described in the Japanese Unexamined Patent Application No. 6-151151 Publication is only for attaining high magnetic permeability in a high frequency range by limiting a main composition range and adding additives (SiO2, CaO, Sb2O2, Nb2O5 and Ta2O5) by suitable amounts. Also, a technique described in the Japanese Unexamined Patent Application No. 9-326310 Publication is only for attaining low losses by limiting a main composition range and adding additives (SiO2, TiO2, CaO, Nb2O5 and Ta2O5) by suitable amounts.
Note that a technique described in the Japanese Unexamined Patent Application No. 6-290926 Publication specifies a composition difference of ZnO between a surface and inside of a sintered core in low-temperature sintering in order to provide a low-loss magnetic material. In the technique described in the Publication, however, it is required that all of SiO2, CaO, TiO2, Nb2O5 and V2O5 are included as additives. However, according to experiments by the present inventors, it was proved that when V2O5 is added as an additives in a composition of the present invention, a microstructure becomes nonuniform and the THD deteriorates adversely.
Also, there described in the Japanese Unexamined Patent Application No. 4-336401 Publication and in the Japanese Unexamined Patent Application No. 10-50512 Publication that it is effective for improving magnetic permeability in a high frequency range by adding Bi2O3. However, according to experiments by the present inventors, it is proved that when Bi2O3 is added as an additive to a composition of the present invention, growth of crystal grain is accelerated, while a microstructure becomes nonuniform and the THD deteriorates adversely.
Note that there were some examples in the past that temperature dependency of a magnetic anisotropy constant K1 became less by adding CoO to an Mnxe2x80x94Zn based ferrite (the Japanese Unexamined Patent Application No. 6-290925, the Japanese Unexamined Patent Application No. 8-191011, the Japanese Examined Patent Application No. 52-4753, the Japanese Examined Patent Application No. 4-33755 and the Japanese Examined Patent Application No. 8-1844 Publications). However, there is no technical idea disclosed of reducing temperature dependency of the THD characteristics of a transformer and reducing the THD of a transformer in a wide temperature range for a magnetic core for transformer of a modem used in high speed data communication of the xDSL by adding CoO.
Namely, all of the Japanese Unexamined Patent Application No. 9-290925, the Japanese Examined Patent Application No. 8-1844 and the Japanese Unexamined Patent Application No. 8-191011 Publications relate to transformers for power sources and objects and use thereof are totally different from those of magnetic cores for transformers of modems. Note that the Japanese Examined Patent Application No. 52-4753 Publication discloses a technique of making a temperature coefficient of magnetic permeability smaller and does not relates to a magnetic core for a transformer of xDSL modems. Since a transmission speed is extremely high in the xDSL, the inductance may be small for increasing the impedance, so the magnetic permeability may be small. Thus, the above Publications disclosing inventions relating to magnetic permeability do not disclose a technique for reducing the temperature dependency of the THD characteristics of a transformer. Also, an invention described in the Japanese Examined Patent Application No. 4-33755 Publication relates to a power source transformer and an inductance element and does not disclose a technique for reducing temperature dependency of the THD characteristics of a transformer.
The present invention has been made in consideration with the above circumstances and has as an object thereof to provide a magnetic core of a transformer preferably used in a modem of xDSL and other high speed data communications with a superior THD characteristics in a broad frequency band and/or wide temperature range and a composition thereof.
To attain the above object, according to a first aspect of the present invention, there is provided an Mnxe2x80x94Zn based ferrite composition having a main component comprised of Fe2O3: 51.5 to 54.5 mol % and ZnO: 19.0 to 27.0 mol % and the rest of substantially MnO, and a first sub component comprised of 0.002 to 0.040 wt % of SiO2, 0.003 to 0.045 wt % of CaO and 0.010 wt % or less of P with respect to 100 wt % of said main component.
By producing a magnetic core of a transformer with the Mnxe2x80x94Zn based ferrite composition according to the first aspect of the present invention, the THD characteristics of the transformer becomes as small as xe2x88x9284 dB or less in a broad frequency band. As a result, distortion of transmission waveforms and noise become small in the xDSL and other high speed data communications, an occurrence of transmission errors can be prevented, and a data signal can be transmitted at high precision.
Note that the THD is defined by the above formula (1).
In the first aspect of the present invention, there is a tendency that when Fe2O3 in the main component composition of the Mnxe2x80x94Zn based ferrite composition becomes less than 51.5 mol %, not only deteriorates the THD but the Curie point falls to an actual use temperature range and properties as ferrite are liable to be lost. While, when Fe2O3 becomes more than 54.5 mol % or ZnO becomes less than 19.0 mol %, the THD is liable to be deteriorated. Also, when ZnO becomes more than 27.0 mol %, the Curie point falls to the actual use temperature range and properties of ferrite are liable to be lost.
In the first aspect of the present invention, when SiO2 becomes out of a range of 0.002 to 0.040 wt %, CaO becomes out of a range of 0.003 to 0.045 wt % or P becomes more than 0.010 wt % in the first sub component, the THD is liable to deteriorate due to abnormal growth of crystal.
According to a second aspect of the present invention, there is provided an Mnxe2x80x94Zn based ferrite composition, having a main component comprised of Fe2O3: 51.5 to 54.5 mol % and ZnO: 19.0 to 27.0 mol % and the rest of substantially MnO, and a first sub component comprised of 0.002 to 0.040 wt % of SiO2, 0.003 to 0.045 wt % of CaO and 0.010 wt % or less of P with respect to 100 wt % of said main component, and a second sub component comprised of at least one selected from 0.050 wt % or less (excluding 0) of Nb2O5, 0.100 wt % or less (excluding 0) of Ta2O5, 0.500 wt % or less (excluding 0) of SnO2, 0.500 wt % or less (excluding 0) of TiO2, 0.500 wt % or less (excluding 0) of In2O3 and 0.150 wt % or less (excluding 0) of Sb2O3 with respect to 100 wt % of said main component.
By producing a magnetic core of a transformer with the Mnxe2x80x94Zn based ferrite composition according to the second aspect of the present invention, the THD characteristics of the transformer become as small as xe2x88x9285 dB or less in a broad frequency band. As a result, distortion of transmission waveforms and noise become small in the xDSL and other high speed data communications, an occurrence of transmission errors can be prevented, and a data signal can be transmitted at high precision.
In the second aspect of the present invention, the main component composition and the first sub component are the same as those in the first aspect of the present invention, and the THD characteristics can be furthermore improved by further containing a second sub component.
Note that when Nb2O5 becomes more than 0.050 wt %, Ta2O5 becomes more than 0.100 wt %, SnO2 becomes more than 0.500 wt %, TiO2 becomes more than 0.500 wt %, In2O3 becomes more than 0.500 wt % or Sb2O3 becomes more than 0.150 wt % in the second sub component, the THD is liable to deteriorate.
In the first and second aspects of the present invention, preferably, the sintered density of the ferrite core is 4.90 g/cm3 or more, an average crystal grain diameter is 7 xcexcm to 40 xcexcm and a composition difference of ZnO between the surface of the sintered core and inside of the sintered core is 1.0 mol % or less.
When the sintered density becomes less than 4.90 g/cm3, the average diameter of crystal grains becomes out of a range of 7 xcexcm to 40 xcexcm, or the composition difference of ZnO between the surface and inside of the sintered core becomes more than 1.0 mol %, the THD is liable to be deteriorated.
Preferably, the first and second aspects of the present invention further have a third sub component comprised of 0.02 to 0.50 wt %, more preferably 0.05 to 0.50 wt %, of Co oxide in CoO conversion with respect to 100 wt % of the main component.
In the first and second aspects of the present invention, the temperature dependency of the THD of a transformer can be made smaller by further containing the third sub component including a Co oxide. Particularly, it becomes possible to reduce the THD (preferably to xe2x88x9284 dB or less) at a normal temperature of around 25xc2x0 C. and reduce the THD of a transformer (preferably to xe2x88x9277 dB or less, more preferably xe2x88x9280 dB or less) stably in a wide temperature range (for example, xe2x88x9240xc2x0 C. to +85xc2x0 C.).
According to a third aspect of the present invention, there is provided an Mnxe2x80x94Zn based ferrite composition, having a main component comprised of MnO: 22.0 to 39.0 mol % and ZnO: 7.5 to 25.0 mol % and the rest of substantially Fe2O3, and a sub component having 0.02 to 0.50 wt % of Co oxide, preferably 0.05 to 0.50 wt % of Co oxide in CoO conversion with respect to 100 wt % of said main component.
By producing a magnetic core of a transformer with the Mnxe2x80x94Zn based ferrite composition according to the third aspect of the present invention, the temperature dependency of the THD characteristics of the transformer can be made small. Particularly, it becomes possible to reduce the THD (preferably to xe2x88x9284 dB or less) at a normal temperature of around 25xc2x0 C. and reduce the THD of a transformer (preferably to xe2x88x9277 dB or less, more preferably xe2x88x9280 dB or less) stably in a wide temperature range (for example, xe2x88x9240xc2x0 C. to +85xc2x0 C.)). As a result, distortion of transmission waveforms and noise become small in an environmental temperature where xDSL modems are used, an occurrence of transmission errors can be prevented, and a data signal can be transmitted at high precision.
In the third aspect of the present invention, the reason, why the temperature dependency of the THD characteristics of a transformer can be made small, can be explained for example as below. Namely, the reason why magnetic losses in an AC magnetic field can be made smaller in a wide temperature range by containing a suitable amount of Co in the Mnxe2x80x94Zn based ferrite composition is considered to be caused by the reason below.
One factor of controlling magnetic losses is magnetic anisotropy constant K1. The magnetic losses changes in accordance with temperature changes of the magnetic anisotropy constant K1 and takes the minimum value at a temperature where K1=0. In order to make the magnetic losses small in a wide temperature range, temperature dependency of the magnetic anisotropy constant K1 has to be small.
The constant differs in accordance with elements constituting a spinel compound which is a main phase of a ferrite. In the case of Mnxe2x80x94Zn based ferrite, the temperature dependency of the magnetic anisotropy constant K1 can be made small and an absolute value of a magnetic-loss temperature coefficient can be made small by introducing Co ion. Namely, the K1 has a negative value in Mnxe2x80x94Zn based ferrite having a composition where Fe2O3 is around 50 mol %, so a temperature range where K1=0 in the Mnxe2x80x94Zn based ferrite can be made wide by containing therein a suitable amount of Co ferrite having a positive K1 value.
In the third aspect of the present invention, there is a tendency that the THD becomes hard to be reduced in a wide temperature range when a Co oxide is less than 0.02 wt % in CoO conversion, while the THD is liable to be deteriorated at a normal temperature of around 25xc2x0 C. when exceeding 0.50 wt %. Therefore, the above range is preferable.
Note that in the third aspect of the present invention, when MnO in the main component composition becomes more than 39.0 mol % or ZnO becomes less than 7.5 mol %, losses in the AC magnetic field becomes large and the THD is liable to be increased.
Also, when MnO becomes less than 22.0 mol % or ZnO becomes more than 25.0 mol %, the Curie point declines to the actual use temperature range and the properties as ferrite are liable to be lost.
In the third aspect of the present invention, preferably, the main component is comprised of MnO: 22.0 to 34.5 mol % and ZnO: 12.0 to 25.0 mol % and the rest of substantially Fe2O3.
In the third aspect of the present invention, preferably, the main component is comprised of MnO: 23 to 33 mol % and ZnO: 13 to 24 mol % and the rest of substantially Fe2O3.
In the third aspect of the present invention, preferably, the main component is comprised of MnO: 23.8 to 24.2 mol % and ZnO: 23.0 to 23.4 mol % and Fe2O3: 52.6 to 53.0 mol %.
In this composition range, the THD characteristics of a transformer can be xe2x88x9285 dB or less in a broad frequency band. Therefore, distortion of transmission waveforms and noise in high speed data communications can be made still smaller, an occurrence of transmission errors can be prevented, and a data signal can be transmitted at high precision.
In the third aspect of the present invention, preferably, the main component is comprised of MnO: 26.1 to 26.5 mol % and ZnO: 20.1 to 20.5 mol % and Fe2O3: 53.2 to 53.6 mol %.
In this composition range, the THD characteristics of a transformer can be xe2x88x9285 dB or less in a broad frequency band. Therefore, distortion of transmission waveforms and noise in high speed data communications can be made still smaller, an occurrence of transmission errors can be prevented, and a data signal can be transmitted at high precision.
In the third aspect of the present invention, preferably, the main component is comprised of MnO: 23.0 to 23.4 mol % and ZnO: 23.4 to 23.8 mol % and Fe2O3: 53.0 to 53.4 mol %.
In this composition range, the THD characteristics of a transformer can be xe2x88x9280 dB or less in a wide temperature range of xe2x88x9240 to +85xc2x0 C. and also in a broad frequency band. Therefore, distortion of transmission waveforms and noise in high speed data communications, such as the xDSL, can be made smaller, an occurrence of transmission errors can be prevented, and a data signal can be transmitted at high precision, even if it is used in an environment of a temperature changing widely.
The above Mnxe2x80x94Zn based ferrite composition according to the present invention is preferably used as a magnetic core for transformers, particularly as a magnetic core of a transformer of xDSL modems.
A method of producing an Mnxe2x80x94Zn based ferrite composition or a magnetic core for a transformer includes the step of sintering the ferrite composition or the magnetic core for a transformer at a sintering holding temperature of 1240 to 1340xc2x0 C. in a sintering process.
In the production method of the present invention, the average diameter of crystal grains of the obtained sintered core tends to become less than 7 xcexcm, the sintered density tends to be less than 4.90 g/cm3, and the THD tends to deteriorates when the sintering hold temperature is less than 1240xc2x0 C. While, when the sintering hold temperature exceeds 1340xc2x0, the composition difference of ZnO between the surface and inside of the sintered core tends to be more than 1.0 mol % and the THD tends to deteriorates.
Note than in the present invention, the holding time of the sintering hold temperature is not particularly limited, but is preferably 3 hours to 10 hours minutes.